1. Field of the Invention
This invention relates in general to servo systems, and more particularly to a notch filter for a disk drive servo system having selectable filtering capability.
2. Description of Related Art
In recent years microcomputer equipment such as personal or desk top computers have become extremely popular for a wide range of business, educational, recreational and other uses. Such computers typically include a main central processor having one or more memory storage disks for storage of data. The storage disk or disks are commonly provided as part of a so-called Winchester disk drive unit, sometimes referred to as a "hard" disk, wherein the storage disks are rotatably supported within a substantially sealed housing. The disks are rotatably driven in unison by a small spindle motor, and one or more electromagnetic heads are displaced by a head actuator assembly to traverse surfaces of the disks for purposes of reading and writing data. Such data is recorded onto magnetizable surfaces or surface films of the disks in the form of data bits located within narrow, closely spaced concentric tracks on the disks. Accordingly, for any given disk drive unit, the total memory storage capacity is directly proportional to the number of disks as well as the number of tracks on each disk.
In normal operations, a system controller of the main central processor has the capability to identify data stored by recording on the disks, typically through the use of directory name, file, name, and/or track address information. When reading of data at a specified data track is desired, the system controller displaces the head or heads to the desired position by supplying appropriate command signals to operate the head actuator assembly. Alternately, when data recording or writing is desired, the system controller operates the actuator assembly to align a selected head with a vacant data track. In either case, the head actuator assembly is commanded to displace the head or heads through a generally radial path relative to the disk for moving each head from alignment with a previously selected track to alignment with the newly selected destination track. Such movement of the head is normally referred to as a "seek" step.
Servo positioning systems have been widely employed for quickly and accurately positioning the mechanical element, such as a magnetic head of a magnetic recording disk apparatus. A typical servo control system for a recording disk apparatus includes a controller for generating a velocity signal based upon a position signal obtained from a servo signal generated by a magnetic head.
The position signal indicates the radial position of the magnetic head relative to the adjacent track and is generally designed to be zero when the head is at the center of a track being traced. The controller calculates the number of tracks needed to be jumped according to command from a host processor unit.
Every servo actuator structure manifests a mechanical resonance at some frequency or frequencies. These vibrational modes, unless accounted for in the servo control design, can result in servo control loop instabilities. Accordingly, the signal from a controller is passed through a notch filter. Typically, a notch filter has an attenuation characteristic such that the attenuation is maximum at the mechanical resonant frequency f.sub.0 thereby filtering out any resonances in the actuator structure in order to stabilize the servo control loop.
It is known that in a discrete time domain (sampling) system, the system control transfer function manifests a minimum value at the sampling frequency. This characteristic has been proposed for use as a notch filter function in a head position servo control loop in order to filter out head actuator resonance, see, e.g., Bauck U.S. Pat. No. 4,398,228 entitled "Method of Avoiding Resonance in Servo Controlled Apparatus". One apparent drawback of this approach is that the sampling frequency must be chosen strictly in relation to the mechanical resonance of the actuator. Such a selection may not be optimum, given other design constraints. For example, when the actuator structure is varied, as by changing the number of disks/heads in the drive, a different sampling frequency and different overall servo control loop design would be required, even within the same product family design. Another inherent drawback is the inability of the Bauck servo loop to sense and respond to e.g. seek arrival transients occurring at the sample frequency and resulting in off-track data writing operations.
The above-described structural dynamics of the actuator can cause servo performance degradation in settle time and random transient vibration (RTV). The dynamics can also cause servo/mechanical instability, which leads to complete performance failure. Often, the dynamics of the actuator is different, but predictable, on different heads in the arm stack due to the vibration mode shape. This causes dynamics problems at different frequencies (different vibration modes) on different heads in the stack.
The mode shapes and frequencies of the various arm "scissor" modes (in-plane bending modes of the arms) can be analyzed to identify the dominant modes. Typically, the dominant modes are in the frequency range of 4-5 kHz. These modes have distinct shapes that cause large vibration motion of some heads and very little motion on others. For example, the "end arm mode" has large motions on the top-most and bottom-most heads in a disk stack for a disk drive having multiple disks. The end arm mode may be higher in frequency than the dominant modes for the inner arms, and the end arms may have no motion at the dominant modes for the inner arms.
Therefore, a single servo notch filter may not be able to improve the performance on all heads. It is also known that a servo notch filter designed to fix a particular mode on a particular head, may cause performance degradation on other heads in the arm stack which have different dynamic characteristics. Further, using a single broad notch in a filter to remove both modes may not work if the frequencies are too different, since a broad notch may adversely effect servo performance.
It can be seen then that there is a need to utilize different servo notch filters for different heads in the actuator head stack.
It can also be seen then that there is a need for a disk drive servo having different notch filters for different heads in the stack.
It can also be seen that there is a need for a disk drive servo that uses one servo notch filter to remove the modes for the internal heads in the stack and a different notch filter for removing the modes for the end heads.